Process of treating sheet steel

ABSTRACT

An improved surface on sheet steel having a carbon content below 0.01%, preferably below 0.003%, for subsequent porcelain enameling or the deposition of a metal or organic, e.g., paint, coating thereon or in preparation of the sheet steel for a subsequent laminating operation, is provided by subjecting the sheet steel having a carbon content below about 0.1% by weight to oxidation in the presence of air under conditions to build up on the surface of the sheet steel undergoing oxidation an oxide layer having a thickness in the range 0.1-1.5 mils and such that the oxidized sheet steel exhibits a gain in weight of at least about 0.5 grams per square foot of oxidized surface, followed by subjecting the resulting oxidized sheet steel to reduction in an atmosphere consisting essentially of nitrogen and hydrogen, preferably at a temperature isothermal with the aforesaid oxidation temperature, such as at a temperature in the range 1000*-1500*F., to reduce substantially all of the oxide layer to metal and then subjecting the resulting oxidized-reduced sheet steel to decarburization at a temperature in the range 1000*1500*F. in an atmosphere consisting essentially of nitrogen, hydrogen and water vapor.

[451 Sept. 16, 1975 United States Patent [191 Uher [ 1 PROCESS OF TREATING SHEET STEEL [75] Inventor: Joseph F. Uher, Timonium, Md.

[73] Assignee: SCM Corporation, Cleveland, Ohio [22] Filed: Mar. 14, 1973 21 App]. No.: 341,259

Related US. Application Data [63] Continuation-in-part of Ser. No. 61,380, Aug. 5,

1970, abandoned.

[52] US. Cl. 427/330; 148/635; 427/399 [51] Int. Cl. C23D 3/00 [58] Field of Search.... 117/49, 53, 118, 129, 135.1; l48/6.3, 6.35, 16, 16.7

[56] References Cited UNITED STATES PATENTS 2,099,340 ll/1937 Kautz 148/635 2,442,485 6/1948 Cook 117/53 2,532,640 12/1950 Pfeiffer 148/167 X 2,878,151 3/1959 Beall et a1 117/53 X 2,956,906 10/1960 Blickwede et al. 1 17/53 X 2,961,337 ll/1960 Bryant et al. 117/53 X 3,043,712 7/1962 Toomey 117/53 X 3,125,471 3/1964 Conner... 148/635 X 3,377,213 4/1968 Hiller 117/118 X 3,389,458 6/1968 Ostrander et a1. 1 17/129 X 3,436,808 4/1969 Kotyk 117/53 X Primary ExaminerThomas J. Herbert, Jr. Assistant Examiner-Bruce 1-1. Hess Attorney, Agent, or Firm-Cooper, Dunham, Clark, Griifin & Moran ABSTRACT An improved surface on sheet steel having a carbon content below 0.01%, preferably below 0.003%, for subsequent porcelain enameling or the deposition of a metal or organic, e.g., paint, coating thereon or in preparation of the sheet steel for a subsequent laminating operation, is provided by subjecting the sheet steel having a carbon content below about 0.1% by weight to oxidation in the presence of air under conditions to build up on the surface of the sheet steel undergoing oxidation an oxide layer having a thickness in the range 0.1-1.5 mils and such that the oxidized sheet steel exhibits a gain in weight of at least about 0.5 grams per square foot of oxidized surface, followed by subjecting the resulting oxidized sheet steel to reduction in an atmosphere consisting essentially of nitrogen and hydrogen, preferably at a temperature isothermal with the aforesaid oxidation temperature, such as at a temperature in the range 1000-1500F., to reduce substantially all of the oxide layer to metal and then subjecting the resulting oxidized-reduced sheet steel to decarburization at a temperature in the range l000-l500F. in an atmosphere consisting essentially of nitrogen, hydrogen and water vapor.

4 Claims, 2 Drawing Figures CO/LED SNE'IY STEFL 65 C Cal/VENT DESIIABLY (0.15

TIME

' TEMP- TIME- PATENTEUSEPI 61975 3,906,125

sum 2 OF 2 SELECT/N6 4 CO/LED STEEL STOCK WHICH W/LL MEET THE END USE REQUIREMENTS AFTER TEEATME/I/T 1 THAT A5 A) CO/LE'D STEEL STOCK T i COA/T4/A/M/6 LESS THAN 0J0? C 6,3,.

AS77 7 SPEC F742568 SHEET STEEL FOR PORCELA/A/ ENAMALM/G I OX/DA 770/1/ 2 TEMP. MM -/500F/A/ 14/1? ATMOSPHERE W/TH M/NOR flMOU/VT OF WATER VAPOR REDUCT/O/V 3 TEMP, 1000 450w N AND H; CONTA/A/WG ATMOSPHERE DECARBUE /z4 770M 4 TEA/IR mad -/5'00"F A z, H; AND

W4 TE)? VAPO/e CO/VTA/A/M/G AT/WJSPHE'PE 5 COOL 1 6 EMPEE/A/G BY COLD POLL/A16 T0 AT LEAST ABOUT 0 570 ELOA/G/QT/OA/ 7 FLASH COAT APPL/CAT/OA/ OF TEMPORHRV 5 PROTECT/V5 comv/vca AND RECON. FOR SH/P/WE/VT PROCESS OF TREATING SHEET STEEL This application is a continuation-in-part of my copending application Ser. No. 61,380 filed Aug. 5, 1970, now abandoned.

This invention in one embodiment relates to the treatment of sheet steel to improve the surface thereof for the subsequent production thereon of a coating of porcelain enamel. In this embodiment the practice of the invention is especially useful when the porcelain enamel coating is a cover coat porcelain enamel directly on the sheet steel since latent gas-type defects in the porcelain enamel during service are substantially eliminated.

This invention in another embodiment relates to the treatment of sheet steel having a carbon content below 0.1% by weight involving the operations, oxidation followed by reduction of the oxidized sheet steel wherein the oxidized layer produced during the oxidation operation is substantially completely reduced to metal with the resultant production on the thus-treated surface of the sheet steel of a surface which exhibits improved properties with respect to a subsequent coating or laminating operation.

Various operations are known for the treatment of sheet steel in order to prepare the surface of the sheet steel for a subsequent coating operation, such as a porcelain enameling operation, see particularly U.S. Pat. Nos. 2,099,340, 2,455,331, 2,602,034 and 2,956,906. The disclosures of these patents are herein incorporated and made part of this disclosure.

For the most part, the techniques and methods known heretofore for the treatment of sheet steel prior to a subsequent coating operation, such as a porcelain enameling operation, have not been completely satisfactory especially when the sheet steel is in coil form as produced at the steel mill. For example, the techniques known heretofore for the treatment of the surface of an enameling steel having a carbon content below 0.01% by weight and suitable for use in a ceramic coil coating operation have not been completely satisfactory. In a ceramic coil coating operation a coil of enameling steel is run through a continuous coating machine, as for example a commercial roll coater or coil coater, wherein finely divided porcelain enamel frit suspended in a curable organic vehicle is applied to the surfaces of the coil, the organic vehicle cured and the resulting coated strip recoiled for shipment to the final fabricator. ln ceramic coil coating the surface of the sheet steel must be prepared for enameling before the application of the coating.

Ordinarily the surface preparation of enameling steel for finish coat porcelain enamels applied directly to the steel includes the operations (1) thorough cleaning of the metal surface; (2) acid pickling to remove rust and scale and to toughen the metal surface and to remove about 2-4 grams of metal per square foot of surface;

(3) a metal flash coating, such as a nickel flash coating of about 0.08-0.15 grams of nickel per square foot of surface; (4) followed by a mildly alkalinebath toneutralize any residual acids or salts and a spray rinse with deionized water. It is, however, presently impractical to treat coiled stock in this manner. For example, a 30 inch wide gauge (0.0359 inch) coil of enameling steel contains approximately 2000 linear feet of steel and weighs 5000 pounds. A suitable treating unit to prepare the surface of such a coil of enameling steel for porcelain enameling, passing the sheet steel through a unit at a minimum speed of 50 feet per minute, would require an. overall metal preparation unit well in excess of 1000 feet in length and would require the sheet to travel through from about 1650 to about 3250 linear feet of solutions in about 12 to 14 different tanks.

It is an object of this invention to provide a method for improving the surface of a coil of sheet steel prior to producing a coating of porcelain enamel thereon.

Another object of this invention is to provide a pro-' cess for improving the surface properties of sheet steel having a carbon content below 0.1% by weight, prior to coating or laminating the sheet steel.

It is another object of this invention to provide an improved process for preparing the surface of a coil of enameling steel having a carbon content below 0.01% prior to subjecting the resulting treated sheet steel to coil coating, such as a coil coating operation wherein a layer of porcelain enamel frit together with a curable organic carrier is deposited as a coating on the sheet steel.

In at least one embodiment of the practice of this invention at least one of the foregoing objects will be achieved. How the objects of this invention are achieved will become apparent in the light of the accompanying disclosure madewith reference to the accompanying drawings wherein:

F IG. 1 is a block diagram which describes a combination of operations included in the practice of this invention; and wherein FIG. 2 is another block diagram showing an overall, more complete combination of operations directed to the treatment of coiled enameling sheet steel such as might be received from a steel mill for the production of a treated coiled sheet steel ready for a coil coating operation.

It has been discovered that the surface of oxidizedreduced-decarburized enameling steel, e.g., an oxidized-reduced-decarburized sheet steel having a carbon content below 0.01% by weight carbon, preferably less than about 0.003% by weight carbon, provides an improved reactive surface for a subsequent coating or laminating operation, such as for a porcelain enameling operation or an organic or metal coating operation. it appears that the resulting oxidized-reduced-decarburized surface is extremely clean, much more so than the cleanliness of a steel surface which might have been cleaned or treated by conventional chemical cleaning methods, such as by chemical cleaning including acid pickling.

1n the combination of operations involving oxidation of the sheet steel followed by reduction and then decarburization, it appears that the oxidation of the surface of the steel produces an oxide layer thereon having a greater volume than the metal making up the oxide layer. During subsequent reduction of this oxide layer to the metal the metal tends to remain in an expanded form, i.e., the metal occupies a substantial portion of the previously formed, expanded oxide layer. The resulting expansion or opening of the surface of the sheet steel as a result of the combination of oxidation and reduction treatments undoubtedly greatly increases the metal available, such as the amount of metal surface available, for interaction with a porcelain enamel in a subsequent porcelain enameling operation.

In the practice of this invention the selection of the steel stock for treatment is important. In general the stock may be any sheet steel of a composition suitable of 'the. sheet steel. Additionally, the carbon content of the starting sheet steel for use in the practice of this invention should desirably be uniform throughout its thickness. Further, the thickness of the sheet steel should desirably be substantially uniform. Sheet steel stock, such as coiled sheet steel, having a thickness in the range 14 gauge (0.075 inch) to about 28 gauge (about 0.015 inch), have been found to be particularly applicable in the practice of. this invention.

To achieve a surface having the superior physical and/or chemical properties obtainable in the practice of this invention, 'it is necessary that the surface of the sheet steel being treated be subjected to the combination of operations, oxidation, reduction and decarburization. FIG. 1 indicates in block diagram form the essential operations of oxidation, reduction and decarburization in the practice of this invention.

The oxidation operation is carried out in the presence of air which may contain a minor amount of steam or water vapor. Satisfactory results are usually achieved by exposing the sheet steel to the oxidizing atmosphere for a period of time in the range from about 5 to about 50 minutesand at a temperature in the range from about l000-1500F., such as ll0O to about 1350F. When the sheet steel is in coil form in accordance with this invention the oxidation operation can becarried out in the same apparatus or chamber previously used for open coil annealing at the mill.

If large coils of sheet steel are subjected to oxidation, such as a steel coil weighing tons more or less, a longer period of time upwards of 60 to about 90 minutes or more may be necessary in order to carry out a satisfactory oxidation operation. Desirably, the oxidation' operation is carried out to produce on the surface of the sheet steel exposed to the oxidizing atmosphere a metal oxide layer having a thickness in the range from about 0.1-0.3 to about 1.0-1.5 mils, although a metal oxide (iron oxide) layer having a somewhat smaller or somewhat greater thickness would also yield satisfactory results. Usually, satisfactory results are obtained when the sheet steel undergoing oxidation exhibits a weight gain in the range about 0.9-3.2 grams per square foot of oxidized surface although a greater or lesser weight gain can be achieved with satisfactory results. A lesser weight gain, e.g. down to about 0.5 grams per square foot, gives satisfactory results and, as indicated, a higher weight gain, e.g., up to about 4.0-7.0 grams per square foot of oxidized surface, also yields satisfactory results, although in such instances the overall strength of the finished, treated sheet steel may be reduced.

Examples of suitable oxidizing atmospheres for use in the practices of this invention, particularly wherein the sheet steel is in open coil form at the mill, are oxidizing atmospheres comprising essentially or substantiallyonly air and steam or water vapor, e.g., 2-30 vol. H O, the remainder being air or oxygen-enriched air up to an 0 content of about 40-50% by vol. Especially satisfactory results are obtained when the oxidation is carried out at l 150F. for a period of time from about 10 to about 70 minutes or for a period of minutes when the oxidation is carried out at about 1350F.

It was observed that the oxide layer'produced in an oxidizing atmosphere containing air and a minor 4 amount of steam at l 150F. appeared to be loose and granular as compared to the preferred tight, adherent oxide layer which formed at about 1350F. The metal oxide coating produced by oxidation in air was tight as was the metal oxide coating produced at 1500F. in an air oxidizing atmosphere.

In the practice of this invention the oxidation temperature, the composition of the oxidizing atmosphere and the time to which the sheet steel is subjected to oxidation are usually adjusted so that there is produced on the surface of the sheet steel being oxidized an oxide layer yielding a weight increment of at least 0.5 gram per square foot of oxidized surface up to a maximum of about 7.0 grams per square foot, as evidenced by an increase in weight of the sheet steel being treated.

Subsequent to the oxidation treatment the oxidized sheet steel is subjected to reduction. Desirably, the reduction operation is carried out substantially isothermally with respect to the oxidation operation, such as at a temperature in the range 1000-1500F., e.g., 1 100l350F.

In the reduction operation a hydrogen-containing gas is employed, such as a gas containing nitrogen and a minor amount of hydrogen, e.g., 15-40% hydrogen, up to a major amount of hydrogen, e.g., 60-90% hydrogen. Gas compositions .which have provided a suitable reducing atmosphere inaccordance with the practices of this invention have comprised about 20% hydrogen and nitrogen. Although, as indicated hereinabove, satisfactory results are obtainable when the reduction operation is carried out at a temperature isothermal with respect to the oxidation operation, if desired, for example to shorten the time required for the reduction operation, the reduction may be carried out at a higher temperature up to about 1500F; At higher temperatures the time required for the reduction of the metal oxide layer with resulting conversion of the metal oxide layer to metal is significantly reduced; for example, the reduction time is shortened from about 60-90 minutes at a temperature of about 1000-l 200F. to about 3-10 minutes at a temperature of about 1500F. Advantageously, at a higher reducing temperature, about 1500F., an improved bonding or sintering of the freshly reduced metal to the unreacted substrate metal is also achieved. Since the reduction temperature is substantially below the melting temperature of the steel, the reaction which produces a layer of essentially pure iron which is in effect a physical extension of the unreacted substrate, is a sintering or solid-state reaction. At higher temperatures, upwards of about 1500F., however, some steels exhibit excessive grain growth. In such a circumstance a lower reducing temperature should be employed.

As indicated hereinabove the oxidation of the sheet steel results in an increase in volume of the sheet steel and an increase in thickness. When, however, the oxide layer produced during the oxidation operation is reduced to metal, the thickness of the sheet steel remains substantially the same. This indicates that the metal making up the surface layer of the sheet steel has been redistributed over an increased volume resulting in a microscopically rough, porous-metal surface having an increased surface area with respect to the surface and metal making up the sheet metal prior to oxidation and reduction' in accordance with this invention.

A suitable decarburization method applicable in the practices of this invention isdescribed in US. Pat. No. 2,956,906. As disclosed therein a suitable decarburization method for the production of low carbon sheet steel, e.g. 0.01% carbon max., involves heating sheet steel having a relatively high carbon content, ,e.g., about 0.060.08% by weight carbon to a temperature in the range l000-l500F., more or less, e.g., in the range 1 l1350F., for an appreciable period of time in a nitrogen-hydrogen containing atmosphere, e.g l0 to 16% hydrogen, remainder nitrogen and some water vapor. Increasing the hydrogen and water vapor content causes the reaction rate to increase, thereby de creasing the time required to reach the desired lower carbon content in the sheet steel undergoing decarburization, e.g., reduction of carbon content down to about 0.003% by weight.

The advantages of the practice of this invention are especially realized when a coil of sheet steel is treated and the treatment is carried out at the mill, i.e., steel mill. The advantages are especially obtainable in this instance since the same chamber or apparatus which might have been employed in any previous treatment of coils at the mill, such as for open coil annealing, might also be used to effect the combination of operations oxidation, reduction and decarburization in accordance with this invention. Furthermore, there could be produced at the mill and directly shipped, by following the practices of this invention, a coil of specialty sheet steel which would be particularly useful for porcelain enameling. Further, if desired, there could be produced at the mill not only a coil of steel having the special surface obtainable by the practice of this invention but also there could be produced at the mill and shipped to the fabricator a coil of sheet steel coated with an organic-frit layer. The fabricator could then fabricate the organic-frit coated sheet steel into a desired shape and after firing produce a porcelain enameled coated finished product.

Further, by carrying out the practices of this invention at the mill where coils of sheet steel are regularly handled and where gaseous streams of varying compositions are readily available one could practice this invention almost under ideal conditions. Specifically, the same apparatus already in place at the mill could also be used to effect in sequence the combination of opera tions oxidation, reduction and decarburization and it would only be necessary to switch from one treating atmosphere to another, the coil remaining in place during treatment. In connection with the oxidation operation a stream of air could be maintained or flowed in contact with the open coil of sheet steel in the container or apparatus where the coil is already in place. Desirably, as indicated hereinabove, the air might contain a controlled, usually minor, amount of steam or water vapor to expedite the oxidation operation. Following the completion of the oxidation operation the oxidizing atmosphere would be displaced by a reducing atmosphere which would be maintained or flowed in contact with the coil steel in the same container or apparatus. Desirably, the reducing atmosphere would be comprised essentially of nitrogen and hydrogen. Although a major amount of hydrogen might make up the reducing atmosphere it is preferred that the hydrogen comprise only a minor amount of the reducing atmosphere to gether with nitrogen, such asan amount of hydrogen not more than about 25% by volume, eg in the range 520% by volume, such as 1 0l-4% by. volume.

Upon completion of the reducing operation the reducing atmosphere, with the open coilof steel in place, would be altered or replaced by a decarburizing atmosphere, preferably made up of nitrogen, hydrogen and water vapor or steam. The reducing atmosphere could readily andconveniently be converted to a decarburizing atmosphere for use in connection with the practice of this invention by the mere addition of steam or water vapor to the reducing atmosphere. Accordingly, by employing the special combination of atmospheres, an oxidizing atmosphere, then a reducing atmosphere and then a decarburizing atmosphere the operations in accordance with this invention for the treatment of a coil of sheet steel to produce thereon a superior surface for subsequent porcelain enameling can readily and easily be achieved at the mill. It would only be necessary to displace the air in making up the oxidizing atmosphere with the reducing atmosphere of nitrogen and hydrogen which then could readily be converted to a decarburizing atmosphere by the mere addition thereto of steam.

After completion of the sequence of operations the resulting treated oxidized, reduced, decarburized coil of sheet steel could be cooled and then tempered as by cold rolling, a conventional steel mill operation, to im prove its strength, followed by application of a flash coating of nickel or cobalt thereto. The resulting treated coil of steel could then be given a temporary, readily removable protective coating of oil and shipped to a ceramic coil coater for the application of a coating of organic-frit and then set to the fabricator. If desired, the coil, after cold rolling and the application of a nickel or cobalt flash coat thereto, could be directly coated at the mill with a curable organic-frit coating which, after curing, could then be shipped directly to the fabricator for fabrication into a desired shape and firing to produce the finished porcelain enamel coated metal object.

Following the oxidation-reduction-decarburization operations the resulting treated sheet steel tends to be soft. It is desirable therefore to temper the oxidizedreduced-decarburized sheet steel, such as by cold rolling, e.g., by cold rolling to an elongation of about 1%, such as an elongation in the range O.52.0%. Even when the treated sheet steel is tempered by cold rolling the improved surface of the tempered treated sheet steel is not substantially affected.

Referring now to FIG. 2 of the drawings, there is indicated therein various embodiments of the practice of this invention directed to the surface preparation of coiled steel stock for porcelain enameling. The combination of steps or stages are indicated in the numeral sequence 1-8.

After the selection of the desired coiled sheet steel stock, Stage 1, an open coil of the steel stock is subjected to oxidation, as indicated in Stage 2, followed by a reduction operation, as indicated in Stage 3, and then a decarburization operation as indicated in Stage 4.

Following the decarburization operation the resulting oxidized, reduced and decarburized sheet steel is cooled, Stage 5, tempered, Stage 6, as by cold rolling to at leastabout 0.5% elongation, such as in the range l2%, to improve hardness and strength, and then coated withaflash coat of nickel or cobalt, Stage 7. In most instances Stage 7 is the preferred position for the application of a flash metal coat to the sheet steel undergoing treatment.

Any suitabletechnique for the deposition of a flash or very thin coating of nickel or cobalt may be employed, such as electrodeposition, electroless deposition, chemical replacement, or metal volatilization and 7 deposition. Usually, an amount of metal, such'as nickel, in the range from about 0.08 to about 0.3 grams per square foot of coated surface is applied. Following the metal coating of the steel stock the steel is desirably one half of the plates were coated directly with a porcelain enamel white cover coat by spraying on an aqueous slip thereof and dried. The resulting coated samples were then fired at I450F. for 3 minutes. The resulting tempered, Stage 7, to improve its hardness, such as by porcelain enamel coating was found defect-free and excold rolling to about 1% elongation. I g l hibited' excellent adherence. The remaining plates were At stage 8, following the temper rolling and flash coated with ceramic frit dispersed in a curable organic coating operations a temporary, readily removable proresin, (a porcelain enamel paint type system used in cetective coating, such as an easily saponifiable oil, e. g., a ramic coil coating). After drying, curing and burning to vegetable oil, may be applied to the steel stock and the remove the organic components from the coating the stock is then recoiled for shipment to the coil coater. coating was fired. There were also produced defect- Upon arrival at the coil coating operation, e.g., a cefree porcelain enamel coatings having excellent adherramic coil coating operation as mentioned hereinence to the steel. above, the coiled treated stock is treated for the re- LE 2 moval of the temporary coating and cleaned in preparation for ceramic coil coating. If desired, the ceramic Samples of sheet steel plates having a carbon content coil coating operation can be carried out at the mill of about 0.003% by weight (meeting ASTM Spec. A with no intervening application of a protective coating. 424, Type I) were subjected to operations involved in The following examples are illustrative of the practhe practice of this invention. These operations are detices of this invention. scribed in accompanying Table I:

TABLE I Nickel Sheet Oxidation Wt. Gain Reduction Reducing coat No. Conditions g/sq.ft. Conditions Atmos. g/sq.ft.

1 15'-1 l00F. 0.6 l0'-l500F. 20% H .112

including steam 80% N2 2 10-1 150F. 0.6 20l500F. 20% H .075

including steam 80% N2 3 75' 1 l50F. 1.5 l0'-l500F. 20% 11 .140

air, no added 80% N2 steam 4 l5-l350F. 1.1 10'l500F. 20% H .148

including steam 80% N2 5 '-1 l50F. 1.3 60-l350F. mostly 1-1 .102

air, no added steam 6 15'-1 15-F. 0.6 60'l350F. 20% H .089

including water 80% N2 vapor 7 '-1 150F. 1.3 90'-l350F. 20% H .078

including steam 80% N2 EXAMPLE 1 Several plates of commercial 20 gauge enameling steel meeting the requirements of ASTM Spec. A 424, Type I and having a carbon content of less than about 0.0030.005% by weight, were weighed and placed in a furnace at l lF. and maintained in air at that temperature for about 45 minutes. There was produced on the steel plates an oxide layer as evidenced by a gain in weight of 1.7 grams per square foot of surface. The oxide layer was found to be about 0.6 mil thick. The oxidized steel plates were then subjected to reduction by being placed in a furnace provided with a hydrogencontaining atmosphere at a temperature of l500F. for a period of 7 minutes. There was no separate decarburizing operation since the steel plates were already decarburized, C content of about 0.003% and since the oxidation and reducing operations already effected some decarburization. The sample plates were then removed and cooled. It was observed that the plates returned to their original weight and the surfaces of the plates were bright, clean and had a satin finish as compared to the very smooth original surface. The plates were then provided with a thin nickel coatingby immersion for 4 minutes in a nickel sulfate solutionmaintained at 170F. and containing about 1 ounce of nickel sulfate per gallon. There was deposited on the resulting treated sample sheets metallic nickel in an amount of 0.055 gram per square foot/After rinsing and 'd'ryine The treated steel plates were then coated with a titania-opacified frit in a conventional clay slip and fired for 3 minutes at about 1460F. The resulting porcelain enamel coatings all exhibited excellent bonding to the underlying metal substrate when tested in accordance with the standard method test for Adherence of Porcelain Enamel and CeramicCoatings to Sheet Metal ASTM C 313-59. Further, additional steel sheets, after oxidation, were reduced at a temperature of 1500F. and coated with a ground coat for porcelain enameling with no nickel flash.'The resulting porcelain enamel coatings, after firing, all exhibited excellent bonding to the underlying substrate and no gas-type defects (e.g., bubbles, blisters, copperheads and fishscales) developedin these coatings during or immediately after firing 'or on aging.

EXAMPLE- 3 Five lots of 20 gauge steel sheet having a carbon content in'the range 0.08-0.09% by weight were subjected.

to oxidation, reduction, decarburization, and nickel flashing in accordance-with the practice of this invention. The carbon content of the resulting treated steel sheets was in the range about 0.0050.007% by weight. The oxidation and reducing operations are indicated in accompanying Table II:

TABLE II Lot Oxidizing Wt. Gain Reducing Reducing No. Conditions glsqft. Conditions Atmosphere 1 45'll50F. 1.50 7-l500F. mostly hyair drogen 2 20'-l 150F. 1.19 l'-l500F. mostly hyair drogen 3 45'1l50F. 1.6 l5'l500F. mostly hyair drogen 4 -ll50F. 1.03 l5-l500F. mostly hyair drogenv 5 45-l150F. 1.58 l5'l500F. mostly hy air drogen The resulting treated nickel coated steel plates were then coated with a titania white porcelain enamel and the porcelain enamel coatings then tested for adherence by impact drop test. The porcelain enamel coatings exhibited good adherence to the underlying steel substrate and no gas-type defects developed. However, when similar nickel-flashed sheet steel stock having a carbon content of about 0.030.05% by weight, i.e., with substantially no decarburization, was enameled with the same porcelain enamel, the porcelain enamel coatings developed gas-type defects.

The practice of this invention is useful in the preparation of coils of sheet steel for porcelain enameling, particularly porcelain enameling involving the production of finish coat porcelain enamel applied directly to the treated sheet steel and especially in such porcelain enamels employing titania-opacified frits.

This invention is particularly useful for the surface preparation of coils of steel sheets suitable for subsequent forming into shapes, such as the steel sheets meeting ASTM Specification A-424-68 (1968) Steel Sheets for Porcelain Enameling, both Type I and Type ll. Desirably, the sheet steel prior to oxidation and reduction should have a very low carbon content below 0.0l% by weight, preferably below about 0.003% by weight, and with no readily discernible carbide area in the grain boundaries. By following the practices of this invention it is possible to apply porce lain enamel cover coats directly to the oxidized reduced-decarburized steel surface following a suitable metal coating, such as nickel fiash treatment. Steels of other compositions, as indicated hereinabove, are also usefully treated in accordance with this invention, such as S.A.E. 1010 steels, and other steels which might also include a minor amount of an alloying metal, such as aluminum.

It would appear that in the preferred commercial practice of this invention, in the embodiment wherein the steel stock to be treated is a coil of steel having a carbon content of about 0.08% by weight, more or less, the coil would be opened as for open coil annealing at the mill. The open coil would then be oxidized, reduced and decarburized, the combination of operations in accordance with this invention. The resulting treated open steel coil would then be closed and the steel cold reduced, as by cold rolling, followed by other operations in accordance with this invention if the resulting treated steel is to be ceramic coil coated directly or shipped for further processing, such as coil coating, e.g., ceramic coil coating, or to a fabricator for the production or fabrication of the finished steel product.

The following example is further illustrative of the practices of this invention, particularly as practiced in a steel mill and involving the treatment of a coil of sheet steel.

EXAMPLE NO. 4

An open coil of sheet steel analyzing about 0.01% C after annealing and in the annealing apparatus or container is subjected to contact with air together with a small amount of steam, the oxidizing atmosphere analyzing about 5% by volume water vapor, the remainder being air. The open coil of steel is maintained in contact with this oxidizing atmosphere at a temperature of 1350F. for 30 minutes. As a result of this oxidation treatment there is produced on the surfaces of the open coil in contact with this oxidizing atmosphere an oxide layer having a thickness of about 1 mil, the coil evidencing an increase in weight due to the formation of this oxide layer of about 3.5 grams per square foot of surface exposed to the oxidizing atmosphere. Upon completion of the oxidation operation, i.e., after 30 minutes, the oxidizing atmosphere is replaced by a reducing atmosphere consisting essentially of nitrogen and hydrogen, hydrogen making up 20% by volume of the reducing atmosphere. The coil is maintained in contact with this reducing atmosphere at a temperature isothermal with the immediately preceding oxidation temperature, 1350F., for 60 minutes, sufficient to substantially reduce all the oxide layer to metal. Upon the completion of this reducing operation, i.e., after the 45 minutes, steam is introduced into the reducing atmosphere to produce a decarburizing atmosphere wherein the steam and hydrogen comprised minor amounts. The decarburizing atmosphere analyzes about 15% by volume hydrogen, 10% by volume steam or water vapor, the remainder being nitrogen. The open coil is maintained in contact with this decarburizing atmosphere at a temperature isothermal with respect to the immediately preceding reducing temperature, i.e., 1350F., for a period of time of about 40 minutes. As a result of this sequence of operations there is produced a coil of sheet steel having a substantially reduced carbon content, i.e., about 0.005% C, and'providing surfaces which are especially desirable for the application thereto of a porcelain enamel coating. Before the porcelain enamel coating is deposited thereon the open coil of steel is closed and then, after cooling, the coil of steel is subjected to a cold rolling (tempering) operation to achieve about 1% elongation. The cold rolling operation increases the hardness and strength of the steel. After the cold rolling operation a flash coating of nickel is deposited on the steel by bringing the steel into contact with an electroless nickel plating bath to deposit on the surface of the steel in contact with the bath an amount of about 0.1 gram of nickel per square foot of surface in contact with the bath. Following the nickel coating operation, and after rinsing and drying, a coating of a curable resin having finely divided porcelain enamel frit dispersed therein is applied to the steel and the organic portion of the organic frit coating cured or dried. The resulting ceramic coil coated steel is then again coiled, after having been cut into desired lengths, and shipped to the fabricator. All the aforesaid operations are carried out at the steel mill. It is thus seen that there is provided in accordance with this invention a sheet steel treatment operation particularly applicable to coiled sheet steel at the mill where the special atmospheres required for oxidation, reduction and decarburization are readily obtained and where the sequence of operations can easily and conveniently be 1 1 coordinated with other steel mill operations, such as annealing.

As indicated hereinabove the unexpected improvement in the surface of the sheet steel making it more receptive for porcelain enameling and the production of an improved porcelain enamel coating thereon, e.g., a porcelain enamel coating which exhibits substantially no gas-type defects during enameling or on aging, appears to be due essentially to the changes in the surface properties of the treated steel surface brought about by the special combination of operations, oxidation, reduction and decarburization carried out in accordance with this invention. Although the practice of this invention is applicable to the preparation of fabricated sheet steel parts prior to porcelain enameling, the practice of this invention is especially applicable to the treatment of sheet stock in coil form and is especially desirable for the treatment of coiled sheet steel at the mill followed immediately by ceramic coil coating, i.e., the application of an organic-frit coating to the oxidizedreduced-decarburized sheet steel. A coil of sheet steel so coated can be fabricated into a desired shape and then fired to produce a superior porcelain enamel coating on the resulting fabricated shape.

Tests to demonstrate the superiority of the practices of this invention with respect to other practices have also been carried out. More particularly,decarburized steel sheets were coated with a white cover coat porcelain enamel of the type regularly used for cover coats directly to decarburized enameling steel, i.e., steel having a carbon content below 0.01%, usually below about 0.003%. Other sheets of decarburized steel were covered with a regular ground porcelain enamel coat. These steel sheets, however, did not receive the surface treatments (e.g., pickling) normally employed in the industry prior to porcelain enameling. The sheets were fired and the porcelain enamel coats were tested for adherence or bonding to the underlying steel by two methods, the method of ASTM C 3 1 359 and the nonstandard test method entitled Drop Weight Adherence Test which is widely used as a control check and which in practice has proved to be more acceptable in the measurement of adherence of cover coats than the above-identified ASTM method. These tests show that the adherence of the porcelain enamel to an untreated decarburized steel sheet was not satisfactory and was poor as compared with the adherence of the porcelain enamel coating to a decarburized steel sheet which had also been oxidized and reduced in accordance with the practices of this invention.

Although emphasis in the disclosure of this invention has been placed on the applicability of the practices of this invention to the preparation of steel sheets prior to aorcelain enameling, the surface of an oxidized, re- :luced and decarburized steel sheet in accordance with :his invention provides an especially satisfactory and ittractive surface for the deposition thereon of a coat- .ng or layer of another material, such as a metal coatng, e.g., aluminum, zinc, chromium, or a coating laid down by a phosphatizing operation or an organic coating, such as plastic, e.g., polyvinyl chloride, polyethylene, chlorinated synthetic rubber, polymerized halogenated hydrocarbon or a polyhalocarbon, such as a polytetrafluoroethylene, a polyvinylidene fluoride, etc., or a paint coating, e.g., a water-base latex paint or oil-base paint or a conversion (thermosetting) varnish or the like. Additionally, the practice of this invention is useful for the preparation of the surface of steel sheet prior to laminating the thus-treated steel sheet, in the presence or absence of added adhesive material, to another surface, such as another metal surface as by cladding or by explosive laminating.

As will be apparent to'those skilled in the art in the light of the foregoing disclosure, many alterations, sub stitutions and modifications are possible in the practice of this invention without departing from the spirit or scope thereof.

I claim:

l. A method of treating an open coil of sheet steel prior to coating said sheet steel with a porcelain enamel coating, said sheet steel having a thickness in the range from about 14 gauge to about 28 gauge and having a carbon content below about 0.1% by weight, which comprises subjecting said coil of sheet steel to oxidation in contact with air at a temperature in the range lOO0l500F. for a period of time sufficient to produce on the surface of said sheet steel exposed to the air ,an oxide layer having a thickness in the range from about 0.1 to about 1.5 mils, the resulting oxidized coil of sheet steel exhibiting a gain in weight in the range from about 0.5 to about 7.0 grams per square foot of surface exposed to the air, subjecting the oxidized coil of sheet steel to a reducing atmosphere consisting essentially of nitrogen and hydrogen at a temperature in the range from about 1000F. to about 1500F. to reduce all of the oxide layer to metal, the aforesaid operations being followed by a decarburizing operation at a temperature in the range 1000l500F. in an atmosphere consisting essentially of nitrogen, hydrogen and water vapor to produce a resulting treated coil of sheet steel having a carbon content below about 0.01% by weight, subjecting said resulting treated coil of sheet steel to cold rolling to at least about 0.5% elongation to effect tempering of the sheet steel and coating the resulting tempered sheet steel with a coating of ceramic frit dispersed in a cured fugitive organic binder.

2. A method in accordance with claim 1 wherein said fugitive organic binder is removed and the ceramic frit is fired to produce on the sheet steel a porcelain enamel coating.

3. A method in accordance with claim 2 wherein the fugitive binder is a curable organic binder and wherein the organic binder is removed by thermal decomposition.

4. A coil of sheet steel prepared by the method of claim 1 and having a COating'Of a ceramic frit dispersed in cured fugitive organic binder. 

1. A METHOD OF TREATING AN OPEN COIL OF SHEET STEEL PRIOR TO COATING SAID SHEET STEEL WITH A PORCELAIN ENAMEL COATING SAID SHEET STEEL HAVING A THICKNESS IN THE RANGE FROM ABOUT 14 GAUGE TO ABOUT 28 GAUGE AND HAVING A CARBON CONTENT BELOW ABOUT 0.1% BY WEIGHT WHICH COMPRISES SUBJECTING SAID COIL OF SHEET STEEL TO OXIDATION IN CONTACT WITH AIR AT A TEMPERATURE IN THE RANGE 1000*-1500*F FOR A PERIOD OF TIME SUFFICIENT TO PRODUCE ON THE SURFACE OF SAID SHEET STEEL EXPOSE TO THE AIR AN OXIDE LAYER HAVING A THICKNESS IN THE RANGE FROM ABOUT 0.1 TO ABOUT 1.5 MILS THE RESULTING OXIDIZED COIL OF SHEET STEEL EXHIBITING A GAIN IN WEIGHT IN THE RANGE FROM ABOUT 0.5 TO ABOUT 7.0 GRAMS PER SQUARE FOOT OF SURFACE EXPOSED TO THE AIR SUBJECTING THE OXIDIZED COIL OF SHEET STEEL O A REDUCING ATMOSPHERE CONSISTING ESSENTIALLY OF NITROGEN AND HYDROGEN AT A TEMPERATURE IN THE RANGE FROM ABOUT 1000*F TO ABOUT 1500*F
 2. A method in accordance with claim 1 wherein said fugitive organic binder is removed and the ceramic frit is fired to produce on the sheet steel a porcelain enamel coating.
 3. A method in accordance with claim 2 wherein the fugitive binder is a curable organic binder and wherein the organic binder is removed by thermal decomposition.
 4. A coil of sheet steel prepared by the method of claim 1 and having a coating of a ceramic frit dispersed in cured fugitive organic binder. 